PROJECT SUMMARY/ABSTRACT Neural crest cells (NCCs) are a transient, multipotent, migratory population of embryonic cells, which give rise to a remarkable array of tissues including the craniofacial skeleton. Disruption of the cellular and molecular mechanisms driving NCC specification, migration, and/or differentiation causes craniofacial disorders representing approximately one-third of all babies born with birth defects. Thus, our long-term goal is to define the NCC gene regulatory network that confers such specialized properties. In this proposal, we will determine whether the transcription factor RONIN (THAP11) directs cranial NCC development by direct transcriptional regulation of cobalamin (vitamin B12) coenzyme biosynthesis. We will also define the contribution of RONIN misregulation to the pathophysiology of a newly discovered human syndrome called cblX. In preliminary studies, we found that Ronin is expressed throughout the developing NCC population and that conditional knockout (CKO) of Ronin caused NCC- derived craniofacial skeletal agenesis without grossly altering other NCC derivatives in the heart, peripheral nervous system and enteric system. These data indicate that the osteochondrogenic NCC lineage is uniquely sensitive to loss of Ronin. We also recently identified Mmachc as a direct transcriptional target of RONIN. MMACHC regulates cobalamin coenzyme production and human mutations in MMACHC cause a combined methylmalonic acidemia and homocystinuria designated cblC. CblC is the most common inborn error of intracellular cobalamin metabolism and often presents as a multisystem disease predominantly impacting hematologic and neurologic development. There are additional reports of craniofacial dysmorphias. Also, mutations in RONIN and its co-factor HCFC1 were recently identified in a cblC-like syndrome, cblX. These data have led to us to hypothesize that the craniofacial defects observed in Ronin mutants are a result of dysregulation of the cobalamin metabolic pathway in cranial NCCs. To test this, we will perform a series of Ronin loss-of-function, genetic interaction, and rescue experiments. We will also determine the functionality of the cobalamin metabolic pathway within wild type and mutant NCCs. Upon successful completion of these aims, we expect to have identified a transcriptional mechanism that links cobalamin metabolism to NCC differentiation into the craniofacial skeleton. This better understanding of NCC development may ultimately inform more efficacious regenerative medicine strategies and lead to new therapeutic targets for craniofacial defects.